A Pokémon appears on the pavement using the augmented reality of Pokémon Go.
Photograph: Sam Mircovich/Reuters

An app that began as an April Fools joke has become a worldwide phenomenon, taking gamers out of the living room and on to the streets as they compete to capture, train and battle Pokémon characters using their mobile phones.

In just seven days since the game was released in the US, Australia and New Zealand, Pokémon Go has now almost certainly exceeded Twitter’s 65 million American users, and the game’s servers have repeatedly crashed under the strain of its popularity.

The share price of Nintendo, which owns a third of the Pokémon Company and an undisclosed stake in the game’s developers Niantic, has rocketed by 50%, and the release of the game in other countries – including the UK – has been put on hold while the developers struggle to cope with demand.

Pokémon Go is a spin-off of the long-running Pokémon series, in which players capture and battle the titular creatures (short for “pocket monsters”) in their quest to become the greatest Pokémon trainer in the world. Unlike in earlier videogames, players move around the real world looking for Pokémon to capture.

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When the app is opened, it shows a map of the area around the player, with various points of interest marked – statues, clock towers and so on. Physically walk up to one of the points, referred to in the game as a Pokéstop, then tap an icon on screen and you’re rewarded with items and experience points.

But the core of the game is the Pokémon themselves, which can appear anywhere, any time (though often themed around the location, with ghost-type Pokémon appearing in graveyards and water-type creatures near lakes and rivers). Tap on one of them and a minigame starts, in which the aim is to catch a Pokémon.

The game is not just for solo players. Some locations have become hubs of Pokémon activity. New York’s Central Park, for instance, has been bustling not just with the usual crowds of sightseers but with players checking their phones for nearby Pokémon.

The real-world aspects of the game have also, inevitably, led to a darker side emerging. Police in Missouri reported that a group of men were waiting near a Pokéstop in a secluded location to target players, and a woman from Wyoming stumbled upon a dead body while trying to reach a water-type Pokemon apparently living in a river by her trailer.

In New Jersey, police were moved to remind residents: “Normally you wouldn’t go to a deserted alley at 3am. That shouldn’t change just because an app said you should.”

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The craze around the game is a huge step up from its humble beginnings as an April Fools joke launched by Google in 2014. For one day, Pokémon appeared on Google Maps, letting browsers search for them at real-world landmarks. “People seemed to really love that combination of Maps and Pokémon,” said Niantic Labs’ chief executive, John Hanke.

But even Hanke couldn’t have predicted the success of Pokémon Go. Data from the analytics firm SimilarWeb suggests that 3% of all Americans opened the Pokémon Go app last Friday, just slightly below the 3.5% who opened Twitter, and the game has already been installed on more American Android phones than the dating app Tinder.

For those in Britain wanting to see what the fuss is about, the best bet is simply to sit tight and wait for the official launch – originally planned to be “a few days” after the US one, but now delayed because of the unexpected capacity issues.

For Niantic and the Pokémon Company, the question is whether they can maintain the immense momentum that Pokémon Go has built up. Some mobile games, such as Candy Crush and Clash of Clans, have used a viral start to build a viable long-term business, but others, including Flappy Bird and Draw Something, have been flash-in-the-plan sensations.

The Pokémon Company has spent 20 years telling players to “catch ‘em all”. Now it has to follow its own advice.